How to cascade (soft) delete has many relationships in GORM?

I am implementing some delete functionality using GORM. My idea is to cascade delete where when the parent row is deleted the childern are deleted aswell. I have tried the delete with select. But it only deletes the parent record.

func DeleteProject(db *gorm.DB, id uint) (Project, error) {
	var project Project
	if err := db.Select(clause.Associations).Delete(&project, id).Error; err != nil {
		return Project{}, err
	}
	return project, nil
}

I have the following model:

type Project struct {
	ID           uint           `gorm:"primary_key" json:"id"`
	CreatedAt    time.Time      `json:"created_at"`
	UpdatedAt    time.Time      `json:"updated_at"`
	DeletedAt    gorm.DeletedAt `gorm:"index" json:"deleted_at"`
	TicketNumber string         `gorm:"size:6;unique;not null" json:"ticketnumber"`
	Name         string         `json:"name"`
	MailingList  string         `json:"mailinglist"`
	Environment  string         `gorm:"not null" json:"environment"`
	UserID       uint           `gorm:"index" json:"user_id"`
	Diagrams     []Diagram      `json:"diagrams"`
}

type Diagram struct {
	ID        uint           `gorm:"primary_key" json:"id"`
	CreatedAt time.Time      `json:"created_at"`
	UpdatedAt time.Time      `json:"updated_at"`
	DeletedAt gorm.DeletedAt `gorm:"index" json:"deleted_at"`
	Data      string         `json:"data"`
	ProjectID uint           `gorm:"index" json:"project_id"`
	Instances []Instance     `json:"instances"`
}

type Instance struct {
	ID        uint           `gorm:"primary_key" json:"id"`
	CreatedAt time.Time      `json:"created_at"`
	UpdatedAt time.Time      `json:"updated_at"`
	DeletedAt gorm.DeletedAt `gorm:"index" json:"deleted_at"`
	Key       uint           `json:"key"`
	DiagramID uint           `gorm:"index" json:"diagram_id"`
}

Where a project can have multiple diagrams and a diagram can have multiple instances. But when I am running the DeleteProject function it is only deleting the Project and not the associated diagrams and instances.

How can I delete a project and it's associated diagrams and instances in this case?

I found an solution, but I think it is not optimal for cascade delete, it solved my problem at least. I've coded the following:

First the DeleteProject function, where I loop over all Diagrams and call the DeleteDiagram function and finally I also clear all associations with this project before deleting the project.

//Delete a project and it's associations.
func DeleteProject(db *gorm.DB, id uint) (Project, error) {
	project, err := GetProject(db, id)
	if err != nil {
		return Project{}, err
	}
	if len(project.Diagrams) > 0 {
		for _, projectDiagram := range project.Diagrams {
			_, err := DeleteDiagram(db, projectDiagram.ID)
			if err != nil {
				return Project{}, err
			}
		}
		db.Model(&project).Association("Diagrams").Clear()
	}
	if err := db.Delete(&project, id).Error; err != nil {
		return Project{}, err
	}
	return project, nil
}

The DeleteDiagram does the same, but then with the Instances

//Delete a diagram and it's associations.
func DeleteDiagram(db *gorm.DB, id uint) (Diagram, error) {
	var diagram, err = GetDiagram(db, id)
	if err != nil {
		return Diagram{}, err
	}
	if len(diagram.Instances) > 0 {
		db.Model(&diagram).Association("Instances").Clear()
		for _, diagramInstance := range diagram.Instances {
			_, err := DeleteInstance(db, diagramInstance.ID)
			if err != nil {
				return Diagram{}, err
			}
		}
	}
	if err := db.Delete(&diagram).Error; err != nil {
		return Diagram{}, err
	}
	return diagram, nil
}

Hope it helps.